Wednesday, February 3, 2016

Lack of Cold and Low Variance

There have been a few mentions recently of the lack of 30-below temperatures so far this winter in Fairbanks; the coldest temperature observed at the airport was -29°F on Christmas Day. If this remains the coldest temperature of the winter, it would be almost unprecedented for lack of cold: in the past, only the winter of 1976-77 failed to reach -30°F (lowest temperature -28°F). The lack of cold is similarly unusual if we look at the 850mb level aloft, where this winter's coldest temperature as measured by balloon sounding was only -14°F on November 18. Only once before, in 2000-2001, did the Fairbanks soundings fail to measure a colder 850mb temperature at some point in the winter.

How likely is it that -30°F will be reached at the airport in the remainder of winter? Based on the last 40 years of historical data, more than 70% of years reach -30°F on or after February 5, and the odds don't drop below 50% until February 22. Even in March it's not too uncommon - in fact, 4 of the last 5 years have seen -30°F or colder in March. However, the medium-range forecast is quite warm (see below), and of course the ongoing El Niño and positive PDO phase suggest that warmth will continue to dominate.

Despite the overall very warm pattern and lack of cold conditions this winter, the number of very warm days has not been particularly unusual. Since November 1, the temperature has risen above freezing on only 5 days in Fairbanks, which is only slightly above the long-term normal of 4 days (in the period Nov 1 - Feb 2), and far below the record of 19 days in 1936-37 (Nov 1 - Feb 2). In other words, the variance of temperature has been low, which is a characteristic of strong El Niño winters.

The chart below shows how the January-March standard deviation of daily mean temperature anomalies (departure from normal) varies with an index of El Niño/La Niña behavior. The standard deviation is quite noticeably reduced when the ENSO index is above +1, so there's little doubt that the El Niño episode is contributing to the lack of variability in Fairbanks this winter.

Fairbanks winter temperature variance is similarly affected by the PDO phase (which is of course correlated with ENSO), and if we create a combined index of PDO+ENSO behavior, the overall correlation with the variance is slightly greater than for ENSO alone.

Here's a chart showing how the ENSO/PDO effects on temperature variance change through the year; each column represents the correlation coefficient for a three-month period. Interestingly the variance reduction for El Niño is slightly greater in late winter than early winter, which probably reflects the fact that El Niño's impacts on the atmospheric circulation reach their peak in late winter. The opposite effect is observed in late summer and autumn, with temperature variance being somewhat enhanced during El Niño conditions and reduced during La Niña.